1. Field of the Invention
The present invention relates to a method for reducing the starting current of a multi-phase machine operated by block commutation.
2. Description of the Related Art
The use of a belt-driven starter generator for starting a motor vehicle is known. This belt-driven starter generator has an inverter situated between the battery of the motor vehicle and the machine. The inverter is an item of power electronics which includes a high side switch and a low side switch. The machine is operable in motor-driven mode and in generator mode.
When starting in motor-driven mode, the start-up of the internal combustion engine of the motor vehicle takes place while the vehicle is idle.
So-called block commutation is a simple and known method for controlling the switches of the power electronics. One advantage of this control method is that it requires no, or at best small, DC-link capacitors. DC-link capacitors of this type may usually be installed in a motor vehicle only with difficulty. A disadvantage of this control method is that high starting currents occur which have a disadvantageous effect on the design of the electronics and the packaging.
The use of PWM at low rotational speeds and block commutation at high rotational speeds is already known from U.S. Pat. No. 7,504,790 B2. This opens up the possibility of limiting the starting current with the aid of high-frequency pulsing. However, the disadvantage of this approach is that a DC-link capacitor having a comparatively high capacitance is needed.
A method for switching between a 120° control and a 180° control is already known from U.S. Pat. No. 6,577,097 B2. Although the need for a high-capacitance DC-link capacitor is eliminated, the problem remains that the starting current is very high in an idle or slowly rotating internal combustion engine, i.e., at a crankshaft speed in the range between 0 revolutions per minute and approximately 400 revolutions per minute. As a result, the load on the output stages is also high. To avoid destroying the output stages due to the high load, the output stages must be adjusted to this high load with the aid of suitable dimensioning. However, this causes the output stages to be overdimensioned for all additional operating points. Another disadvantage is that undesirable voltage drops may occur in the power supply to the vehicle electrical system, due to the high currents flowing during the startup phase. In block-commutated systems, there is no way to reduce the high currents mentioned above with the aid of high-frequency pulsing, for example at 16 KHz, since no DC-link capacitor having a sufficient capacitance is available for this purpose.